The invention disclosed and claimed herein generally pertains to a shield for a magnet. More particularly, the invention pertains to a passive magnetic shield for use with the type of magnet employed in magnetic resonance imaging or spectroscopy to generate the main magnetic field.
As is well known, magnetic resonance imaging requires the use of a magnet for generating a very strong magnetic field, which may be on the order of 1.5 Tesla, in a hospital or like environment. Such magnets, sometimes referred to as main magnets, typically comprise a number of magnetic coils supported along a cylindrical frame around a bore which is of sufficient size to receive a human patient. In some arrangements, the coils are maintained at an extremely low temperature so that the conducting material forming the coils operates in a superconducting mode. External power requirements for a main magnet are thereby minimized.
The strong magnetic field generated by the main magnet includes a fringe field, or field component extending into the space around the magnet. The fringe field may be strong enough to interfere with the operation of electronic devices such as pacemakers or other equipment of a medical nature which must be operated in the vicinity of the magnet. Accordingly, various practices have been followed or proposed to prevent the fringe field from exceeding a threshold level beyond a specified distance from the main magnet. Five Gauss has typically been selected as the threshold value, and the term "Five Gauss line" refers to a boundary within which the fringe field exceeds 5 Gauss.
In some hospitals, adverse effects of fringe fields are avoided by lining a room containing an MRI system with steel shielding, or by locating the system in its own specially constructed building. Such approaches tend to involve great cost. In other approaches to limiting fringe fields, shielding material is placed around the main magnet, in close proximity thereto. One such arrangement is shown in U.S. Pat. No. 4,758,812, issued July 19, 1988 to Forster et al. Forster discloses several alternative configurations of such magnet shields, respectively formed of beams, rods or plates. However, in some of such arrangements the shielding elements may have to be incorporated as an integral part of the main magnet. Because such elements tend to be bulky, heavy, or of odd design, magnet construction costs are significantly increased. Also, such arrangements cannot be used to provide shielding for already existing magnets, and may adversely affect the homogeneity of the magnetic field within the magnet bore, thereby introducing errors into the imaging process.
As a further drawback, prior-art shielding systems of the above type may impose non-uniform forces on the respective coils of a magnet. Thus, the coils are urged out of alignment with one another, so that their respective axes are no longer in colinear relationship. In a superconducting coil, the fields of misaligned coils tend to interfere with each other such that the coils heat and lose their superconductivity, or "quench." To prevent quenching, prior-art systems, as exemplified by Forster et al., provide devices for individually adjusting coils to ensure proper alignment. However, the adjusting process may be very tedious, and of limited effect.